Mathcad really shows its power when you begin to use variables. To assign a value to a variable, use the assignment operator.
To enter a variable, type in the variable name.
Now, click the assignment operator button and type in "4".
We assigned x the value of 4 using the assignment operator. 
The assignment operator is different from the equal sign, also on the symbol palette.
Now let's define y in terms of x.
To evaluate y, type "y" and click on the equal button in the symbol palette. You could also press the equal sign on your keyboard.
Where you place your math on the document can be important.
To see why, we'll move one of the math equations.
You can select an equation by clicking and dragging the mouse across it. A dashed selection box appears.
Now click on the equation and drag it to move it anywhere on the document.
That's interesting! The message bar states that the variable is undefined.
That's because Mathcad reads from top to bottom and left to right. We need to define x before using it in the equation for y.
We need to move x:=4 above the equation for y.
Now Mathcad can evaluate y.
Can I use a variable to represent more than one number?
You can use a range variable instead, that will calculate x for all the values in your range.
To create a range variable, type the variable name and click on the assignment operator.
Then, type in the first value of the range, followed by a ",".
Type in the next value in the range, and click on the range variable button.
Finally, type in the last value of the range. You will then have a range variable that goes from 0 to 4 in steps of .5
If you type "x =", you will see all the values of the range.
If the step between each value is 1, then we could just give the endpoints of the range. Let's try this for the variable y.